#20: Tower Raiders Gold

Publisher: Gianormous GamesPrice: $4.99

Whatever the platform, whatever the price range, tower defense games are killing it. They're all pretty similar: you spend money on towers to kill off successive waves of enemies. Killing enemies gets you more cash to spend on more or better towers.

#19: Space Physics

Publisher: Camel GamesPrice: $1.99

Ignore the incredibly boring name, this is one of the better reviewed Android games ever. Each level has you moving an object from one place to another, getting past obstacles with tools you draw with your finger (and the laws of physics). Think Scribblenauts for adults.